Significant advances in recent years in the reduction in physical size and cost of electronic circuits have resulted from improvements in manufacturing techniques such as microlithography. In general, microlithography comprises applying a film of radiation sensitive material, i.e. a resist medium, to the surface of a substrate, irradiating a selected portion of the resist layer with a source of irradiation, e.g., ultraviolet light, an electron beam, a laser beam, X-rays or the like, and developing the film with a solvent to remove a portion thereof thus forming a relief pattern. For a positive resist medium, the portion irradiated will be dissolved in the developer solvent whereas the nonirradiated portion will be dissolved for a negative resist medium. The resist material remaining on the surface is utilized as a protective mask to facilitate the selective etching or other treatment of the exposed portion of the underlying substrate.
In general, the good sensitivity possessed by negative resists is more than offset by the loss of both resolution and adhesion to the substrate resulting from development with organic solvents. In addition, a high erosion rate, often exceeding fifty percent, is common with wet development of negative electron beam and X-ray resists. Also, the development of such resists commonly leaves a residual scum on the exposed portion of the substrates which must be removed, conventionally, in an oxygen plasma. The high erosion plus the descumming step often leave only very thin layers of resist so that pinhole defects become a substantial problem.
In accordance with this invention, there are provided negative resist structures which are particularly useful with irradiation means, such as electron beam and X-rays, which, when utilized with conventional negative resists, are characterized by substantial erosion during development. The subject two-layer resist system is ideally suited for direct write irradiation, particularly by electron beam.